Advancement of our knowledge on Aso volcano:
Current activity and backgroundEditorial Board (October 2020)
Editor-in-Chief
Yasuo Ogawa, Tokyo Institute of Technology, Japan

Vice Editors-in-Chief
Masato Furuya, Hokkaido University, Japan
Solid Earth Science

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Space Science

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Yoshimori Honkura
Toshihiko Iyemori

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Earth, Planets and Space(2019) 71:105
Ohkura et al. Earth, Planets and Space
https://doi.org/10.1186/s40623-019-1083-x

Open Access

PREFACESpecial issue “Advancement of ourknowledge on Aso volcano: current activity
and background”
Takahiro Ohkura1*, Akihiko Yokoo1, Yasuo Miyabuchi2, Jerry Fairley3, Corrado Cigolini4 and Valerio Acocella5

Introduction
Aso volcano is one of the most active volcanoes in Japan
located in the volcanic front on Kyushu-Ryuku Island arc.
Central cones including the active Nakadake cone locatedinside the caldera which was formed by four pyroclastic
flow eruptions. The Nakadake cone hosts a hot and acidcrater lake and frequent phreatic-to-magmatic eruptions.
From gigantic caldera-forming eruptions to fumarolicactivities, Aso volcano provides us with a wide-range of
research subjects which improve our understandings ofthe volcanic system.
The most recent magmatic eruptions at Aso volcano
characterized by Strombolian explosions and continuous ash venting, began on 25 November 2014, after about
20 years of dormancy. In late 2015 and 2016, violent phreatic and phreatomagmatic explosions occurred repeatedly. This special issue focuses on scientific researches
at Aso volcano completed before, during, and after the
2014–2016 eruptive activities. In total, 19 articles were
published in the special issue; these are described accordingly with the investigating methods.

and lithic grains. In the November 25–27, 2014 ash-falldeposits, lithic fragments, which are interpreted to be
derived from lavas or pyroclasts of previous eruptions,were dominant (59–68%). Thereafter, the proportion ofglass shards, which are probably juvenile materials of
newly ascending magma, gradually increased with time,
and the December 21–23, 2014 ash contained abundant
glass grains (63%). The proportions of glass shards ranged
from 29 to 50% until February 25, 2015. Subsequently,
they decreased with time and reached 14% on March 17.
Afterward, the proportions increased again prior to April
27 and ranged between 20 and 30% in May 2015. From
November 25, 2014 to the end of January 2015, the cumulative erupted mass increased at a high discharge rate
(2.2 × 104 tons/day). After February 2015, the cumulative
erupted mass decreased to a low rate of 0.6 × 104 tons/
day, although this rate rose slightly in March and late
April 2015. The total erupted tephra mass was 2.1 × 106
tons (1.2 × 104 tons/day), which was less than the tephra
deposits of previous activities that have occurred within
the past few decades.
Namiki et al. (2018) conducted three series of measurements for high-porosity scoriae and ash ejected fromthe Nakadake crater during magmatic eruption in 2014–2015. From their results of measurements, Namiki et al.
(2018) elucidated the eruptive conditions causing suchash emission and generation of scoriae and inferred thesequence of an eruption as follows. Cooling of the uppermost part of the high-porosity foam in the conduit causes
fracturing of the magma foam, which creates the glassy
brown ash to be ejected. Some part of the brown ash fallsback onto the top of the high-temperature foam and isaltered to black ash. The ash particles are initially mobile,and the ash layer has a large permeability. Ultimately,

Geology, petrology, and material sciences
In the special issue, four articles are published on thegeological, petrological, and mineralogical aspects oferuptions at Aso. A frontier letter by Miyabuchi and Hara(2019) presents the distribution, discharged mass, andcomponents of tephra-fall deposits during the 2014–2015
eruption of the Nakadake first crater at Aso volcano. The
tephra-fall deposits consisted of glass shards, crystal,*Correspondence: bonkura@aso.vgs.kyoto‑u.ac.jp
1
Kyoto University, Kyoto, Japan
Full list of author information is available at the end of the article© The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License(http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium,provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license,
and indicate if changes were made.

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4I-1 melt, indicating that they were incorporated into themelt immediately prior to eruption; and (3) amphibole
phenocrysts crystallized at a depth of ~ 13.9±3.5 km,
which coincides with the depth of the present low-velocity zone beneath the volcano, implying that the depthof the post-caldera magma plumbing system is strongly
influenced by a relic collapsed magma reservoir related tothe most recent caldera-forming eruption.

the bottom of the ash layer may undergo sintering andits permeability is reduced. The underlying foam layer
is pressurized and ash eruption occurs. Once the debris
layer at the top of the magma foam is removed, theunderlying magma foam erupts as scoriae.
In Saito et al. (2018), petrological observations andchemical analyses on scoriae and related melt inclusions
in mineral phases were used to investigate the magma
ascent and eruption processes of the 1979, 1989, and 2014
eruptions of Nakadake crater, Aso volcano. The wholerock analyses of the scoria produced by the November
26–27, 2014 eruption indicated that they are andesite incomposition and identical to those of the 1979 and 1989
eruptions. Thermometry using the chemical composition of the groundmass and the rims of the phenocrysts
indicated that the temperature of the 2014 magma was
1042–1092 °C. Melt inclusions in plagioclases, clinopyroxenes, and olivines in the 2014 scoria had an andesite
composition similar to that of the groundmass. The volatile content of the melt inclusions was 0.6–0.8 wt% H2O,
0.003–0.017 wt% CO2, and 0.008–0.036 wt% S. The variation in CO2 and S content of the melt inclusions was
not correlated with the K2O content, suggesting that themagma degassed as pressure decreased. Melt inclusions
in plagioclases, clinopyroxenes, and olivines from the1979 and 1989 scoria had similar major elements and volatile content to the 2014 eruption specimens. The similarity in chemical composition of both the whole-rockand melt inclusions among all samples suggests that themagmas of these eruptions were derived from the same
magma chamber. The gas saturation pressure estimatedfrom the H2O and CO2 contents of the 1979, 1989, and2014 scoria ranged from 18 to 118 MPa, corresponding
to depths of 1–4 km. Based on the bulk sulfur content
of the magma and the SO2 flux between January 2014and December 2017, the amount of degassed magma
over that period was estimated to be the equivalent of
1–3 km3 of dense rock. The estimated volume was morethan 600 times larger than that of eruptive products during the same period. This suggests degassing of magma inthe chamber associated with magma convection within aconduit.
Ishibashi et al. (2018) investigated chemical compositions of amphibole phenocrysts in pyroclasts collectedfrom the initial and largest pyroclastic unit(4I-1) of the
Aso-4 caldera-forming eruption occurred at ~ 90 ka toclarify their crystallization conditions and pre-eruptive
magmatic processes. The results suggest the following: (1)most of the amphibole phenocrysts coexisted with silicate melt with 66–72 wt% SiO2 and temperatures of 910–
950 °C, whereas some amphiboles crystallized from more
mafic and higher-T melt; (2) the amphibole phenocrysts
are in thermal and chemical disequilibrium with the hostGeophysical research
There are seven articles in the special issue on geophysical studies. In Kanda et al. (2019), AMT data acquired
between 2004 and 2005 were reanalyzed using a recently
developed 3-D inversion code incorporating detailed
topography into the model in order to estimate thedetailed shallow resistivity structure of Aso volcano. A
highly conductive zone is observed beneath the active
crater down to a depth of approximately 300 m. Basedon the recent findings regarding the shallow hydrothermal system of the volcano such as Ichimura et al. (2018),
this conductive zone could be interpreted as formed byhighly conductive acidic fluids filling a fractured region.
This view modifies the past interpretation made basedon the 2-D models and promotes understanding of fluid
behavior beneath the active crater. And this 3-D model
was used as an initial model in Minami et al. (2018).
Huang et al. (2018) derived crustal Vs structures
beneath Aso caldera using analyses of ambient seismic
noise signals. Daily CCFs of seismic station pairs were
calculated and then CCFs were stacked monthly, toobtain Rayleigh wave phase velocity dispersion curves.
And 1–5-s phase velocity maps were constructed with
0.05° grid spacing. The post-caldera central cones arecharacterized by high velocities from the surface to adepth of 1 km. In the center of the post-caldera central
cones, low velocities prevail at the surface and extend to
major anomalies at depths of 1–2.5 km. These low-velocity anomalies can be assumed to be shallow hydrothermal
reservoirs that might be related to surface geothermal
activity. The low velocities identified at depths of 5–6 km
beneath the post-caldera central cones might indicate the
top of magma chambers. The low-velocity belts situatedat 2.5–5 km depths are likely pathways for the transfer ofhydrothermal fluids, volcanic gases, or melting magma to
the surface.
Ichimura et al. (2018) estimated the source locationsof continuous tremors at Aso volcano over a 2-month
period preceding ash–gas emissions in January 2014.
During the period from December 2013 to January 2014,
a significant variation in the amplitude of continuousseismic tremors corresponding to surficial volcanic activity was observed around the Nakadake crater. The tremor
source locations were estimated by a three-dimensional2

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times. The obtained depth was < 400 m and the ascentvelocity of the gas phase was estimated to be 1–160 m/s.
This velocity is too fast to assume the migration of a gas
slug through the conduit. The ascent velocity of a slugfrom theoretical and experimental approaches cannot
exceed 7.5 m/s under the conditions present at Aso volcano. To explain the estimated values, they proposed arevised model describing the migration of the gas phase
via a more complicated mechanism, such as annular flow.
Yokoo et al. (2019) provides detailed descriptionsof monochromatic infrasound waves observed at Aso
volcano. Throughout the entire eruption period in
2014–2015, when both ash venting and Strombolian
explosions occurred, monochromatic infrasound waves
were observed nearly every day. The source location of
the signals was highly stable at the active vent. Although
the peak frequency of the signals (0.4–0.7 Hz) changed
over time, the frequency exhibited no reasonable correlation with the eruption style. Based on finite-differencetime-domain modeling using 3-D topographic data of
the crater during the eruption (March 2015), the propagation of infrasound waves from the conduit was calculated assuming that the shape of the conduit was a simple
pipe. The peak frequency of the observed waveforms waswell reproduced by the calculation, while the length ofthe pipe markedly defined the peak frequency. By replicating the observed waveform, the gas exhalation with agas velocity of 18 m/s occurred at 120 m of depth in the
conduit.

grid search using the tremor amplitude ratio of 5–10 Hz
band-pass filtered waveforms. The estimated source locations were distributed in a roughly cylindrical region(100–150 m in diameter) ranging from the ground surface to a depth of 400 m just beneath the crater. Migration of the estimated source location was also identified
and was associated with changes in volcanic activity.
Minami et al. (2018) reported results of ACTIVE during the previous magmatic eruption period of Aso volcano (November 2014 to May 2015). 3-D inversion using
a finite-element method of the ACTIVE data sets from
August 2014 and August 2015 succeeded in revealing
temporal variations in the resistivity structure between
the period before and that after the magmatic eruptions
as follows: a noticeable decrease in resistivity at an elevation of ~ 1050 m on the western side of the crater, and anincrease in resistivity at elevations of 750 to 850 m, notonly below the crater bottom but also extending outsideof the crater. The increase in resistivity can be ascribed toa decrease in the amount of conductive groundwater inthe upper part of an aquifer located below the elevationof 800 m, while the decrease in resistivity implies thatenhanced fluid temperature and pressure changed thesubsurface hydrothermal system and formed a temporal
fluid reservoir at the shallow level during the magmatic
eruption period.
Tsunematsu et al. (2019) conducted a video camera
observation of the Strombolian eruptions at Aso volcano
in 2015 and analyzed the video images to investigate thegas flow effect on the particle transport of large pyroclasts (> 10 cm). Using the obtained trajectory data, thefeatures of Strombolian activity such as ejection velocity,
explosion energy, and particle release depth were investigated. The gas flow velocities were estimated by comparing the simulated and observed trajectories. The rangeof the ejection velocity of the observed eruptions was
5.1–35.5 m/s, while the gas flow velocity, which is larger
than the ejection velocity, reached a maximum of 90 m/s,
with mean values of 25–52 m/s for each bursting event.
The particle release depth, where pyroclasts start to moveseparately from the chunk of magmatic fragments, wasestimated to be 11–13 m using linear extrapolation of
the trajectories. Although these parabolic trajectories
provide us with an illusion of particles unaffected by thegas flow, the parameter values show that the particles aretransported by the gas flow, which is possibly released
from inside the conduit.
Ishii et al. (2019) analyzed characteristic VLP signals,
eruption earthquake signals, and infrasound signals
accompanying Strombolian explosion events at Aso volcano in late April 2015. The explosion depth and ascentvelocity of a gas phase in the conduit were estimatedusing the differences in the seismo-acoustic signal arrival

Geochemical research
Three articles on geochemical research were publishedin this special issue. A frontier letter by Shinohara et al.
(2018a), reports chemical compositions of the salt fallouts observed during the intensive gas emission andash eruption stages of the Nakadake crater of the Aso
volcano. Spherical hollow salt shells were observed on
several occasions during and shortly after the weak ash
eruptions. Most of the salt fallouts have composition
similar to those of the dried crater lake water samples
and are quite different from those of the ash leachates.
The hollow structure of the shells suggests that they wereformed by the heating of hydrothermal solution droplets suspended by a mixed stream of gas and ash in theplume. The salt shells indicate a close distribution of thehydrothermal system surrounding the erupting vent,even during the continuous magmatic eruption stage.
Morita et al. (2019) conducted continuous monitoringof soil CO2 flux in the flank of Nakadake cone, Aso volcano, from January 2016 to November 2017. After applying a multivariate linear regression analysis, the obtained
time series of soil CO2 flux presented some anomalous peaks in both the active and calm periods. Careful

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whereas ground-based measurements initially were fluctuating around 1 MW. They provided VRP threshold values that define the transition from high fumarole activity,
through phreatic–phreatomagmatic activity, to a Strombolian phase at active Nakadake crater.
Morita (2019), using land observation data of theplume height and satellite observation data of SO2 mass,reports on the degassing activity before the 8 October
2016 phreatomagmatic eruption of Aso volcano. In thisstudy, the temporal variations of the plume height, theSO2 mass, and ground-based SO2 flux during 6 months
before the eruption were investigated. The results showthat the maxima and the increasing trends in the above
parameters, respectively, occurred about 2 months and6 days before the eruption. This result indicates that thedegassing system had been stable during 6 months before
the eruption and that the accumulation of volcanic gas
in the conduit, since August might trigger the phreatomagmatic eruption. It was shown that these techniques
can be effective to monitor the degassing activity of the
volcanoes.
Ishii et al. (2018) used the Ash RGB images from Himawari-8 to detect and track SO2 cloud from a phreatomagmatic eruption of Aso volcano on October 8, 2016.
The Ash RGB is a composite image of three observation
bands of Himawari-8 (RED: brightness temperature difference between 12.4 and 10.4 μm, GREEN: brightness
temperature difference between 10.4 and 8.6 μm, andBLUE: 10.4 μm), and can discriminate SO2 clouds andvolcanic ash clouds from meteorological clouds. They
could estimate the height of the SO2 cloud by comparing
the Ash RGB images and simulations of the JMA Global
Atmospheric Transport Model: the estimated height of
the SO2 cloud was 7–13 or 7–14 km. Using the cloudheight and eruption duration, the total mass of volcanic
ash from the eruption was estimated to be 6.1–11.8 × 108kg, which is consistent with 6.0–6.5 × 108 kg obtained
from field observations.
Weather radars of JMA also captured the volcanic ash
cloud from the explosive eruption of Aso on October 8,2016 (JST). Sato et al. (2018) used five radars’ data andfound that the ash cloud travel more than 200 km awayfrom the crater. The eruption altitude was estimated tobe 12,000 m ASL ± 687 m (1σ). Using this result and theduration of the earthquake (160–220 s) due to the eruption, the total mass of the ejecta was estimated to be 3.2–
7.5 × 108 kg, which is consistent with the value of the fieldsurvey.

comparison of the anomalous peaks with the environmental parameters revealed that most of the anomalous
peaks were likely due to an increase in wind speed and/or
a decrease in barometric pressure. However, the anomaly
after the 8 October 2016 eruption could not completely
be explained by the variations in the environmental
parameters and coincided with increases in seismic
amplitude and plume SO2 flux. This anomaly was possibly attributed to an increase in magmatic CO2 flux. Thesefindings emphasized the importance of careful statisticaltreatment of the soil CO2 flux data by excluding the influences of the environmental parameters at each measurement site.
Shinohara et al. (2018b) reported gas composition
measurement obtained by means of multi-GAS, which
was collecting data during the 2014–2015 eruptive phase,
as well as the quiet period preceded the above events.
Volcanic gas composition during the eruptive period ischaracterized by rapid and large variation. In particular,
the CO2/SO2 and SO2/H2S varied in the rages of 1–8 and
3–300 during the ash eruption with intermittent Strombolian activity. The variation shows a well-constrainedcorrelation between two end-member compositions: one
is CO2/SO2 = 8 and SO2/H2S = 3 and the other is CO2/SO2 = 1.5 and SO2/H2S = 300. The large variation and thenegative correlation in compositions are attributed to amarked difference in the degassing pressure with reaches
two orders of magnitude, such as 20 and 0.2 MPa; thegases with the large CO2/SO2 and the small SO2/H2S are
related to high-pressure conditions. The rapid and large
compositional variation suggests frequent ascent of bubbles formed at various depth during the eruption. The
maximum CO2/SO2 decreased with the intensity of theeruption suggesting a decline of the bubbles derived froma large depth. With time, H2O/SO2 increases from 30 to> 60, suggesting an increase in a hydrothermal contribution at lower depth.

Remote sensing
Four articles in the special issue are based on remotesensing analysis. Cigolini et al. (2018) analyzed the thermal signature of Nakadake crater of Aso volcano during
unrest episodes by combining the MODIS-MIROVA dataset (2000–2017) with high-resolution images (LANDSAT
8 OLI and Sentinel 2) together with ground-based thermal observations (2013–2017). During fumarolic activity, VRP detected by both of the satellite and groundobservation was below 0.5 MW and reached 2–2.8 MWwith increasing activity. Prior, during, and after the
major Strombolian explosions, satellite VRP data areabove 10 MW, reaching peak values of 15.6 MW. Afterthe volcano re-entered the phreatic phase, satellite dataprocessed by MIROVA exhibit very few thermal alerts,

Modeling
Data assimilation methods were applied to estimate
the mass of eruption plume column. Ishii (2018) developed a data assimilation system based on the 4D-Var for4Earth, Planets and Space
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estimating the mass of eruptive columns as a functionof altitude and ash particle size and applied this method
to analyze the October 8, 2016 eruption of Aso volcano.
Using both field observation on ash fall and meteorological radar observations for the data assimilation system,emission mass from the eruption plume column (as afunction of altitude and particle size) was estimated andsubsequent modeling was consistent with observations.
The total mass of eruptive column was estimated to be
1.32 × 108 kg.

Ishii K (2018) Estimation of emission mass from an eruption plume for theAso volcano eruption, on October 8, 2016, using a four‑dimensionalvariational method. Earth Planets Space 70:202. https://doi.org/10.1186/
s40623‑018‑0964‑8
Ishii K, Hayashi Y, Shimbori T (2018) Using Himawari‑8, estimation of SO2 cloud
altitude at Aso volcano eruption, on October 8, 2016. Earth Planets Space70:19. https://doi.org/10.1186/s40623‑018‑0793‑9
Ishii K, Yokoo A, Kagiyama T, Ohkura T, Yoshikawa S, Inoue H (2019) Gas flow
dynamics in the conduit of Strombolian explosions inferred from seismo‑
acoustic observations at Aso volcano, Japan. Earth Planets Space 71:13.
https://doi.org/10.1186/s40623‑019‑0992‑z
Kanda W, Utsugi M, Takakura S, Inoue H (2019) Hydrothermal system of theactive crater of Aso volcano (Japan) inferred from a three‑dimensional
resistivity structure model. Earth Planets Space 71:37. https://doi.
org/10.1186/s40623‑019‑1017‑7
Minami T, Utsugi M, Utada H, Kagiyama T, Inoue H (2018) Temporal variation in
the resistivity structure of the first Nakadake crater, Aso volcano, Japan,
during the magmatic eruptions from November 2014 to May 2015, as
inferred by the ACTIVE electromagnetic monitoring system. Earth Planets
Space 70:138. https://doi.org/10.1186/s40623‑018‑0909‑2
Miyabuchi Y, Hara C (2019) Temporal variations in discharge rate and com‑ponent characteristics of tephra‑fall deposits during the 2014–2015
eruption of Nakadake first crater, Aso Volcano, Japan. Earth Planets Space
71:44. https://doi.org/10.1186/s40623‑019‑1018‑6
Morita M (2019) Temporal variations of plume activities before the 8 October
2016 eruption of Aso volcano, Japan, detected by ground‑based and sat‑
ellite measurements. Earth Planets Space 71:7. https://doi.org/10.1186/
s40623‑019‑0986‑x
Morita M, Mori T, Yokoo A, Ohkura T, Morita Y (2019) Continuous monitoringof soil CO2 flux at Aso volcano, Japan: the influence of environmental
parameters on diffuse degassing. Earth Planets Space 71:1. https://doi.
org/10.1186/s40623‑018‑0980‑8
Namiki A, Tanaka Y, Yokoyama T (2018) Physical characteristics of scoriae andash from 2014–2015 eruption of Aso Volcano, Japan. Earth Planets Space70:147. https://doi.org/10.1186/s40623‑018‑0914‑5
Saito G, Ishizuka O, Ishizuka Y, Hoshizumi H, Miyagi I (2018) Petrological char‑acteristics and volatile content of magma of the 1979, 1989, and 2014
eruptions of Nakadake, Aso volcano, Japan. Earth Planets Space 70:197.
https://doi.org/10.1186/s40623‑018‑0970‑x
Sato E, Fukui K, Shimbori T (2018) Aso volcano eruption on October 8, 2016,
observed by weather radars. Earth Planets Space 70:105. https://doi.
org/10.1186/s40623‑018‑0879‑4
Shinohara H, Geshi N, Yokoo A, Ohkura T, Terada A (2018a) Salt shell fallout dur‑ing the ash eruption at the Nakadake crater, Aso volcano, Japan: evidenceof an underground hydrothermal system surrounding the erupting vent.
Earth Planets Space 70:46. https://doi.org/10.1186/s40623‑018‑0798‑4
Shinohara H, Yokoo A, Kazahaya R (2018b) Variation of volcanic gas composi‑tion during the eruptive period in 2014–2015 at Nakadake crater, Asovolcano, Japan. Earth Planets Space 70:151. https://doi.org/10.1186/s4062
3‑018‑0919‑0
Tsunematsu K, Ishii K, Yokoo A (2019) Transport of ballistic projectiles during
the 2015 Aso Strombolian eruptions. Earth Planets Space 71:49. https://doi.org/10.1186/s40623‑019‑1029‑3
Yokoo A, Ishii K, Ohkura T, Kim K (2019) Monochromatic infrasound waves
observed during the 2014–2015 eruption of Aso volcano, Japan. Earth
Planets Space 71:12. https://doi.org/10.1186/s40623‑019‑0993‑yAbbreviations
4D‑var: four‑dimensional variational method; ACTIVE: a controlled‑source
electromagnetic volcano monitoring experiment; AMT: audio‑frequency
magnetotelluric; CCFs: cross‑correlation functions; JMA: Japan Meteorologi‑
cal Agency; JST: Japan standard time (UTC + 9); VLP: very long period; VRP:
volcanic radiative power.
Acknowledgements
We are very grateful to the authors who submitted papers to this special issue
and the many referees who have given their time and provided us with valu‑
able suggestions.
Authors’ contributions
TO drafted the manuscript. AK, YM, JF, CC and VA improved the manuscript. All
authors read and approved the final manuscript.
Competing interests
The author declares that they have no competing interests.
Author details
1
Kyoto University, Kyoto, Japan. 2 Kumamoto University, Kumamoto, Japan.
3
University of Idaho, Idaho, USA. 4 University of Torino, Torino, Italy. 5 Roma Tre
University, Roma, Italy.
Received: 18 September 2019 Accepted: 18 September 2019

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5Advancement of our knowledge on Aso volcano: Current activity and background

FRONTIER LETTER

Open Access

Salt shell fallout during the ash eruption at the Nakadake crater,
Aso volcano, Japan: evidence of an undergroundhydrothermal system surrounding the erupting vent
Hiroshi Shinohara*, Nobuo Geshi, Akihiko Yokoo, Takahiro Ohkura and Akihiko Terada
Earth, Planets and Space 2018, 70:46 DOI:10.1186/s40623-018-0798-4
Received: 1 September 2017, Accepted: 6 February 2018, Published: 20 March 2018

Abstract
A hot and acid crater lake is located in the Nakadake crater, Aso volcano, Japan. The volume of water in the lake decreases withincreasing activity, drying out prior to the magmatic eruptions. Salt-rich materials of various shapes were observed, falling fromthe volcanic plume during the active periods. In May 2011, salt flakes fell from the gas plume emitted from an intense fumarole
when the acid crater lake was almost dry. The chemical composition of these salt flakes was similar to those of the salts formed by
the drying of the crater lake waters, suggesting that they originated from the crater lake water. The salt flakes are likely formed by
the drying up of the crater lake water droplets sprayed into the plume by the fumarolic gas jet. In late 2014, the crater lake driedcompletely, followed by the magmatic eruptions with continuous ash eruptions andintermittent Strombolian explosions. Spherical hollow salt shells were observed on
several occasions during and shortly after the weak ash eruptions. The chemicalcomposition of the salt shells was similar to the salts formed by the drying of thecrater lake water. The hollow structure of the shells suggests that they were formedby the heating of hydrothermal solution droplets suspended by a mixed stream ofgas and ash in the plume. The salt shells suggest the existence of a hydrothermal
system beneath the crater floor, even during the course of magmatic eruptions.
Instability of the magmatic–hydrothermal interface can cause phreatomagmatic
explosions, which often occur at the end of the eruptive phase of this volcano.

Keywords: Aso volcano, Hydrothermal system, Salt, Fallouts, Ash leachate, Volcanic
eruption

Graphical abstract*Corresponding author: Hiroshi Shinohara, shinohara-h@aist.go.jp

FRONTIER LETTER

Open Access

Temporal variations in discharge rate and component characteristics
of tephra-fall deposits during the 2014–2015 eruption of
Nakadake first crater, Aso Volcano, Japan
Yasuo Miyabuchi* and Chihoko Hara
Earth, Planets and Space 2019, 71:44 DOI:10.1186/s40623-019-1018-6
Received: 7 November 2018, Accepted: 21 March 2019, Published: 15 April 2019

Abstract
The 2014–2015 eruption of the Nakadake first crater at Aso Volcano in southwestern Japan was characterized by continuous ash
emissions and intermittent strombolian eruptions. In this paper, we present the distribution, discharged mass, and components oftephra-fall deposits to examine the sequence of activities. We installed 21 ash samplers around the crater (SW crater rim to 9 km in all
directions) and calculated the mass of ash-fall deposits based on 28 isomass maps. From November 25, 2014, to the end of January
2015, the cumulative erupted mass increased at a high discharge rate (2.2 × 104 tons/day). After February 2015, the cumulative
erupted mass decreased to a low rate of 0.6 × 104 tons/day, although this rate rose slightly in March and late April 2015. The 2014–2015 tephra-fall deposits consisted of glass shards, crystal, and lithic grains. In the November 25–27, 2014 ash-fall deposits, lithic
fragments, which are interpreted to be derived from lavas or pyroclasts of previous eruptions, were dominant (59–68%). Thereafter,
the proportion of glass shards, which are probably juvenile materials of newly ascending magma, gradually increased with time, andthe December 21–23, 2014 ash contained abundant glass grains (63%). The proportions of glass shards ranged from 29 to 50% until
February 25, 2015. Subsequently, they decreased with time and reached
14% on March 17. Afterward, the proportions increased again prior to
April 27 and ranged between 20 and 30% in May 2015. The total
erupted tephra mass from the November 2014–May 2015 activity of
Nakadake first crater was 2.1 × 106 tons (1.2 × 104 tons/day), which wasless than the tephra deposits of previous activities that have occurred
within the past few decades.

Keywords: Erupted mass, Component characteristics, Tephra-fall deposit,
Small-scale eruption, Nakadake first crater, Aso Volcano

Graphical abstract*Corresponding author: Yasuo Miyabuchi, miyabuchi@gmail.com
6

Earth, Planets and Space

FULL PAPER

Open Access

The thermal signature of Aso Volcano during unrest episodes
detected from space and ground-based measurements
Corrado Cigolini*, Diego Coppola, Akihiko Yokoo and Marco Laiolo
Earth, Planets and Space 2018, 70:67 DOI:10.1186/s40623-018-0831-7
Received: 9 November 2017, Accepted: 5 April 2018, Published: 26 April 2018

Abstract

distance (m)The thermal signature of Aso Volcano (Nakadake) during unrest episodes has been analyzed by combining the MODIS-MIROVA data set (2000–2017)with high-resolution images (LANDSAT 8 OLI and Sentinel 2) and ground-based thermal observations (2013–2017). The site of major activity (crater
1) is located at the summit of the volcano and is composed by a fumarole field (located in the South Area) and an acidic lake (replaced by a Central Pit
during Strombolian phases). The volcanic radiative power (VRP) obtained by nighttime satellite data during the reference period was mainly below 3
MW. This thermal threshold marks the transition from high fumarole activity (HFA) to Strombolian eruptions (SE). However, periods characterized bysporadic phreatic eruptions (PE, eventually bearing phreatomagmatic episodes), which is the prevalent phase during unrest episodes, exhibit very
low VRP values, being around 0.5 MW, or below. The statistical analysis of satellite data shows that the transition from HFA to Strombolian activity(which started on August 2014 and ceased in May 2015) occurs when VRP values are above the cited 3 MW threshold. In particular during marked
Strombolian phases (November–December 2014), the radiative power was higher than 4 MW, reaching peak values up to 15.6 MW (on December 7,
2014, i.e., 10 days after the major Strombolian explosion of November 27). Conversely, ground-based measurements show that heat fluxes recordedby FLIR T440 Thermo-camera on the fumarole field of the South Area has been relatively stable around 2 MW until February 2015. Their apparent
temperatures were fluctuating around 490–575 °C before the major Strombolian explosive event, whereas those recorded at the active vent, named
Central Pit, reached their maxima slightly above 600 °C; then both exhibited a decreasing trend in the following days. During the Strombolian activity,
the crater lake dried out and was then replenished by early July, 2016. Then, volcanic activity shifted back to phreatic–phreatomagmatic and theeruptive cycle was completed. During this period, the MIROVA system detected very few thermal alerts and the ground-based measurements werefluctuating around 1 MW. The most violent explosion occurred on October-600 -300
0300
600
8, 2016, and within the following weeks measured VRP were moderately
600
23/11/2013
25/10/2014
11/08/2016
24/04/2017aabove 2 MW. This is coeval with a thermal increase at the fumarole field of300
centralthe South Area, with temperatures well above 300 °C. Thermal monitoring
crater
south
pit
south
south
lake
0
at Aso Volcano is an additional tool in volcano surveillance that may
area
area
area
contribute to near-real-time hazard assessment.
-300

Keywords: Aso Volcano, Unrest episodes, Fumarolic activity,

HFA

PE

SE

Strombolian activity, Major explosions

HFA

Graphical abstract*Corresponding author: Corrado Cigolini, corrado.cigolini@unito.it

FULL PAPER

Open Access

Temporal variation in source location of continuous tremors
before ash–gas emissions in January 2014 at Aso volcano,Japan
Misa Ichimura, Akihiko Yokoo*, Tsuneomi Kagiyama, Shin Yoshikawa and Hiroyuki Inoue
Earth, Planets and Space 2018, 70:125 DOI:10.1186/s40623-018-0895-4
Received: 22 February 2018, Accepted: 26 July 2018, Published: 3 August 2018

Abstract

SO2 (tons/day)

RMS amplitude ( m/s)

Volcanic tremor is often observed to be associated with an increase in volcanic activity and during periods approaching eruptions. It
is therefore of crucial importance to study this phenomenon. The opening of a new vent and subsequent ash–gas emissions wasobserved in the active crater (Nakadake crater) of Aso volcano, Japan, in January 2014. These events were considered to be associatedwith phreatomagmatic activity similar to the small events of 2003–2005. During the period from December 2013 to January 2014, a
significant variation in the amplitude of continuous seismic tremors was observed corresponding to surficial volcanic activity. We
estimated the tremor source locations for this two-month period by a three-dimensional grid search using the tremor amplitude ratioof 5–10 Hz band-pass filtered waveforms. The estimated source locations were distributed in a roughly cylindrical region (100–150 m
in diameter) ranging from the ground surface to a depth of 400 m. Migration of the estimated source location was also identified andwas associated with changes in volcanic activity. We assumed that the source locations coincided with a conduit system of the
volcano, consisting of networks of fractures. This area is likely situated
6
2500
above the crack-like conduit proposed in previous studies. Before the52014 event, an increase in gas-dominated volcanic fluid first caused
2000
an enlargement of the conduit zone, followed by the migration of
4
1500
further magmatic fluid through other pathways, which resulted in a3
subsequent ash–gas emission. Although we do not have sufficient
1000
30 Octinformation to discuss the causal relationship between these2
processes, it seems reasonable that continuous tremors might change
500
1
the conduit conditions.
001-Nov

Keywords: Aso volcano, Volcanic continuous tremors, Source
location, Pathway of volcanic fluids

01-DecGraphical abstract*Corresponding author: Akihiko Yokoo, yokoo.akihiko.5a@kyoto-u.ac.jp
701-Jan001-Feb

Advancement of our knowledge on Aso volcano: Current activity and background

FULL PAPER

Open Access

Amphibole–melt disequilibrium in silicic melt of the Aso-4caldera-forming eruption at Aso Volcano, SW Japan
Hidemi Ishibashi*, Yukiko Suwa, Masaya Miyoshi, Atsushi Yasuda and Natsumi Hokanishi
Earth, Planets and Space 2018, 70:137 DOI:10.1186/s40623-018-0907-4
Received: 29 March 2018, Accepted: 11 August 2018, Published: 23 August 2018

Abstract
The most recent and largest caldera-forming eruption occurred at ~ 90 ka at Aso Volcano, SW Japan, and is known as the “Aso-4 eruption.”
We performed chemical analyses of amphibole phenocrysts from Aso-4 pyroclasts collected from the initial and largest pyroclastic unit
(4I-1) of the eruption to infer the composition–temperature–pressure conditions of the melt that crystallized amphibole phenocrysts.
Each amphibole phenocryst is largely chemically homogeneous, but inter-grain chemical variation is observed. Geothermometry,geobarometry, and melt–SiO2 relationships based on amphibole single-phase compositions reveal that most amphibole phenocrysts
were in equilibrium with hydrous melt comprising ~ 63–69 wt% SiO2 (SiO2melt) at 910–950 °C, although several grains were crystallized
from more mafic and higher-temperature melts (~ 57–60.5 wt% SiO2 and 965–980 °C). The amphibole temperatures are comparable withthose previously estimated from two-pyroxene geothermometry, but are much higher than temperatures previously estimated from
Fe–Ti oxide geothermometry. The estimated SiO2melt contents are lower than that of the host melt in the 4I-1 pyroclasts. Chemical andthermal disequilibrium between the amphibole rims and the host melt, as well as
intra-grain homogeneity and inter-grain heterogeneity of amphibole compositions,suggests that these amphiboles were incorporated into the host melt immediately prior
to the caldera-forming eruption. Our results suggest that the amphibole phenocrysts,
and perhaps some of the pyroxene and plagioclase phenocrysts, were derived from achemically and thermally zoned crystal mush layer that had accumulated beneath thechamber of the host 4I-I melt. Amphibole geobarometry indicates a crystallization depthof ~ 13.9 ± 3.5 km, which is consistent with the present-day magma chamber depth
beneath the volcano as inferred from geophysical observations. The results suggest thatthe depth of the post-caldera magma plumbing system is strongly influenced by a relicmagma reservoir related to a previous caldera-forming eruption.

Keywords: Amphibole, Aso Volcano, Caldera, Crystal mush, Thermobarometry

Graphical abstract*Corresponding author: Hidemi Ishibashi, ishibashi.hidemi@shizuoka.ac.jp

FULL PAPER

Open AccessPhysical characteristics of scoriae and ash from 2014–2015
eruption of Aso Volcano, Japan
Atsuko Namiki*, Yukie Tanaka and Tadashi Yokoyama
Earth, Planets and Space 2018, 70:147 DOI:10.1186/s40623-018-0914-5
Received: 28 March 2018, Accepted: 21 August 2018, Published: 15 September 2018

Abstract
The activity at Aso Volcano was mainly defined as a sequence of ash emissions and occasional ejections of scoria fragments with ash. Ashemissions sometimes started without notable explosions. The measured porosity of scoriae was as high as 0.94. The scoriae had a flattened shapewith a low-porosity outer rim. To elucidate the eruptive conditions causing such ash emission and generation of scoriae, we conducted threeseries of measurements. First, we heated the high-porosity scoriae from Aso Volcano at 900–1150 °C and found that the heated scoriae shrunk bylosing the gas in the bubbles. At the highest temperature, 1150 °C, bubbles segregated from the surrounding melt. Second, we conducted shear
deformation experiments of scoriae and ash at 500–950 °C and found that the high-porosity scoriae easily fractured by low normal and shear
stresses of ~ 104Pa at a low temperature of 500 °C. We also found that the fine ash at a high temperature of 950 °C was sintered. Third, we
measured the permeability of the sintered ash plate and unheated powder-like ash layer. The permeability of the ash plate is less than
2.5×10−13m2, while that for the ash powder is greater than 10−11m2. The unheated ash particles could move in the container during thepermeability measurements. This effect allowed the formation of pipe-like structures in the ash layer and increased its permeability. On the basisof these measurements, we infer the conditions inside the erupting conduit. There exists high-porosity magma foam in the conduit. The top of
the magma foam is cold (<500 °C) and has a sufficiently high porosity(>0.7) to be fractured at a low stress level (~ 104Pa). The fractured magma
foam generates the ash layer above the magma foam. The gas flow fromthe underlying magma foam makes the high-permeability structure in
the ash layer. Eventually, the bottom of the ash layer sinters to regulate
the gas flow. The pressurized magma foam breaks the sintered ash layer.
The breakage at the bottom of the ash layer may not cause a notable
explosion but causes ash emission. The fragmented magma foam
becomes high-porosity scoriae at a high temperature, which cangenerate the low-porosity outer rim by shrinkage and flatted shape.
ScoriaGas

Ash

Gas flow

0.1MPa0.1MPa0.1MPa0.1MPak>10-11m2

Temperature

0.2MPak<10-13m2

0.2MPa20MPa

0.2MPaPressure>1000 oC
Temperature
Pressure

Keywords: Foam, Rheology, Friction, Fracturing, Permeability>1000 oC

>1000 oC

>1000 oC

Temperature
Pressure

Temperature
Pressure

Temperature
Pressure

Graphical abstract*Corresponding author: Atsuko Namiki, namiki@hiroshima-u.ac.jp
8Earth, Planets and Space

FULL PAPER

Open Access

Variation of volcanic gas composition during the eruptive
period in 2014–2015 at Nakadake crater, Aso volcano, Japan
Hiroshi Shinohara*, Akihiko Yokoo and Ryunosuke Kazahaya
Earth, Planets and Space 2018, 70:151 DOI:10.1186/s40623-018-0919-0
Received: 28 March 2018, Accepted: 5 September 2018, Published: 21 September 2018

Abstract
Volcanic gas composition measurement by Multi-GAS was repeated during the eruptive period in 2014–2015 as well as the quiet
period preceding the eruption at Nakadake crater, Aso volcano. The eruptive activity is characterized by continuous ash emission
with intermittent Strombolian activity and temporal pauses. Volcanic gas composition measured during the eruptive period
showed a rapid and large variation. In particular, the CO2/SO2 and SO2/H2S ratios varied in the rages of 1–8 and 3–300 during theash eruption with a clear negative correlation. The large variation and the negative correlation of the compositions are attributed
to two orders of magnitude difference of degassing pressure, such as 20 and 0.2 MPa; the gases with the large CO2/SO2 and thesmall SO2/H2S ratios are derived from the high pressure. The rapid and large composition variation suggests frequent ascent of
bubbles formed at various depth during the eruption. The maximum CO2/SO2 ratio decreased with decreasing eruption intensitythat suggests decrease in contribution of the bubbles derived from a large depth. With time, H2O/SO2 ratio of the gases increases
from 30 to > 60, suggesting increase in a hydrothermal contribution.

Keywords: Aso volcano, Eruption, Volcanic gas, Multi-GAS,
Degassing

Graphical abstract*Corresponding author: Hiroshi Shinohara, shinohara-h@aist.go.jp

FULL PAPER

Open Access

Shallow volcanic reservoirs and pathways beneath Aso
caldera revealed using ambient seismic noise tomography
Yu-Chih Huang*, Takahiro Ohkura, Tsuneomi Kagiyama, Shin Yoshikawa and Hiroyuki Inoue
Earth, Planets and Space 2018, 70:169 DOI:10.1186/s40623-018-0941-2
Received: 30 March 2018, Accepted: 20 October 2018, Published: 29 October 2018

Abstract
Imaging the shallow velocity structures beneath Aso caldera is necessary to further understand volcanism at the volcano and in the region.
The network for monitoring Aso volcano has been progressively renewed and upgraded with denser and more modern instruments. We usedapproximately 4 years of seismic data recorded by a network of 25 seismic stations to image S-wave velocity (Vs) structures beneath Aso
caldera with seismic noise interferometry. We calculated daily cross-correlation functions (CCFs) of
broadband and short-period station pairs separately and then stacked CCFs monthly, to obtain thetime-domain empirical Green’s functions and corresponding Rayleigh-wave phase-velocity dispersion
curves. Finally, we constructed 1–5-s phase-velocity maps interpolated from nodes spaced 0.05° grid.
The maps allow investigating crustal Vs structures between the surface and a depth of 6 km, likely
related to shallow volcanic reservoirs and pathways. High velocities are found within the first kilometerof the crust beneath post-caldera central cones. Low velocities in the center of the post-caldera central
cones extend from the surface to a depth of 1–2.5 km; we infer that the anomalies mark shallowhydrothermal reservoirs likely replenished by precipitation and hydrographic networks. The
prevalence of high velocities below 3 km can be considered as consolidated igneous rock. Lowvelocity anomalies identified at depths of 5–6 km beneath the post-caldera central cones could be amanifestation of magma accumulation. The low-velocity belts situated at 2.5–5 km depths are likely
pathways for the transfer of hydrothermal fluids, volcanic gases, or melting magma to the surface. The
northern part of the caldera shows relevant lateral velocity variations, with low velocities and highvelocities predominant in the east and west, respectively. Other low-Vs anomalies appear near thesurface to the west and northwest of Aso caldera.

Keywords: Ambient seismic noise, S-wave velocity structure, Aso caldera, Shallow crust, Volcanic
reservoirs and pathways

Graphical abstract*Corresponding author: Yu-Chih Huang, huangyc@ncree.narl.org.tw
9Advancement of our knowledge on Aso volcano: Current activity and background

FULL PAPER

Open AccessPetrological characteristics and volatile content of magmaof the 1979, 1989, and 2014 eruptions of Nakadake,Aso volcano, Japan
Genji Saito*, Osamu Ishizuka, Yoshihiro Ishizuka, Hideo Hoshizumi and Isoji Miyagi
Earth, Planets and Space 2018, 70:197 DOI:10.1186/s40623-018-0970-x
Received: 21 March 2018, Accepted: 5 December 2018, Published: 14 December 2018

Abstract
Petrological observations and chemical analyses of melt inclusions in scoria were used to investigate the magma ascent and eruption processes of
the 1979, 1989, and 2014 eruptions of Nakadake, Aso volcano, Japan. Major elements and sulfur contents of the melt inclusions were determinedusing an electron probe microanalyzer, and their water and CO2 contents were determined using secondary ion mass spectrometry. Five scoria
specimens from the 2014 eruptions had an andesite composition identical to the scoria from the 1979 and 1989 eruptions. Thermometry using thechemical composition of the groundmass and the rims of the phenocrysts indicated that the temperature of the 2014 magma was 1042–1092 °C.
Melt inclusions in plagioclases, clinopyroxenes, and olivines in the 2014 scoria had an andesite composition similar to that of the groundmass. The
volatile content of the melt inclusions was 0.6–0.8 wt% H2O, 0.003–0.017 wt% CO2, and 0.008–0.036 wt% S. The variation in CO2 and S content of themelt inclusions was not correlated with the K2O content, suggesting that the magma degassed as pressure decreased. Melt inclusions in plagioclases,clinopyroxenes, and olivines from the 1979 and 1989 scoria had similar major elements and volatile content to the 2014 eruption specimens. The
similarity in chemical composition of both the whole-rock and melt inclusions among all samples suggests that the magmas of these eruptions were
derived from the same magma chamber. The gas saturation pressure estimated from the H2O and CO2 contents of the 1979, 1989, and 2014 scoria
ranged from 18 to 118 MPa, corresponding to depths of 1–4 km. Comparison of this depthwith geophysical observations suggests that the inclusion entrapments occurred in the
0.05
0.05
a
b
upper part of the magma chamber and/or a conduit. By combining the melt inclusion
0.04
0.04
analysis with volcanic gas observations, we estimated the bulk volatile content of themagma. Based on the bulk sulfur content of the magma and the SO2 flux between January
0.03
0.03
2014 and December 2017, the amount of degassed magma over that period was estimated
3
0.02
0.02
to be the equivalent of 1–3 km of dense rock. The estimated volume was more than 600
times larger than that of products erupted during the same period. This suggests that
0.01
0.01
magma degassing occurred at several depths in the magma chamber due to magma
0.00
0.00
convection in a conduit.
0.00
0.01
0.02
0.03
0.04
0.05
0
1
2
3
Volcanic gas-B
Volcanic gas-A(2015.1.12)(2015.1.12)
CO2/H2O=0.2 (in mol) CO2/H2O=0.04 (in mol)

CO2 (wt %)150MPaB3 A3 B1

A1

Volcanic gas-B(2015.1.12)
CO2/SO2=10 (in mol)

CO2/(H2O+CO2)
in gas (in mol)

0.8

0.7

Volcanic gas-A(2015.1.12)
CO2/SO2=1 (in mol)

0.6

B2 A2

0.5

100MPa0.4

50MPa

CO2 (wt %)

0.9

0.3
0.2

25MPa

0.1

10MPa

Keywords: Aso volcano, Nakadake, Magma, Eruption, Magma ascent, Degassing,
Melt inclusion, Volatile, Magma plumbing system

S (wt %)H2O (wt %)Graphical abstract*Corresponding author: Genji Saito, saito-g@aist.go.jp

FULL PAPER

Open AccessContinuous monitoring of soil CO2 flux at Aso volcano, Japan:
the influence of environmental parameters on diffuse
degassing
Masaaki Morita*, Toshiya Mori, Akihiko Yokoo, Takahiro Ohkura and Yuichi Morita
Earth, Planets and Space 2019, 71:1 DOI:10.1186/s40623-018-0980-8
Received: 31 August 2018, Accepted: 21 December 2018, Published: 7 January 2019

Abstract
Continuous measurements of soil CO2 flux are useful for understanding degassing processes and for monitoring volcanic activities. Recent
studies at many volcanoes have revealed that soil CO2 flux variations are significantly influenced by environmental parameters as well as
volcanic processes. In this study, we conducted continuous monitoring of soil CO2 flux in the flank of Nakadake cone, Aso volcano, Japan, fromJanuary 2016 to November 2017. The results of our observations during an active period before and after a large phreatomagmatic eruption
on 8 October 2016 and during a calm period from 2017 showed variations in soil CO2 flux due to oscillations in environmental parameters.
Excluding these variations from the raw time series by multivariate linear regression
analysis, the time series of soil CO2 flux presented some anomalous peaks in both theactive and calm periods. Careful comparison of the anomalous peaks with theenvironmental parameters revealed that most of the anomalous peaks were likely due
to an increase in wind speed and/or a decrease in barometric pressure. However, theanomaly after the 8 October 2016 eruption was not completely explicable by thevariations in the environmental parameters and coincided with increases in seismic
amplitude and plume SO2 flux. This anomaly was possibly attributed to an increase inmagmatic CO2 flux. These findings emphasized the importance of careful statisticaltreatment of the soil CO2 flux data after excluding the influences of the environmental
parameters at each measurement site. These statistical treatments will contribute to abetter understanding of the degassing processes and monitoring of volcanic
activities, including phreatic or phreatomagmatic eruptions.

Keywords: Aso volcano, Soil CO2 flux, Accumulation chamber method, Multiple
linear regression analysis, Environmental parameters

Fig. 9*Corresponding author: Masaaki Morita, aki.morita@aist.go.jp
10Earth, Planets and Space

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Open Access

Monochromatic infrasound waves observed during
the 2014–2015 eruption of Aso volcano, Japan
Akihiko Yokoo*, Kyoka Ishii, Takahiro Ohkura and Keehoon Kim
Earth, Planets and Space 2019, 71:12 DOI:10.1186/s40623-019-0993-y
Received: 30 August 2018, Accepted: 24 January 2019, Published: 31 January 2019

Abstract
Monochromatic infrasound waves are scarcely reported volcanic infrasound signals. These waves have the potential to provide constraints on
the conduit geometry of a volcano. However, to further investigate the waves scientifically, such as how the conduit shape modulates the
waveforms, we still need to examine many more examples. In this paper, we provide the most detailed descriptions of these monochromatic
infrasound waves observed at Aso volcano in Japan. At Aso volcano, a 160-day-long magmatic eruption occurred in 2014–2015 after a 20-yearquiescent period. This eruption was the first event that we could monitor well using our infrasound network deployed around the crater.
Throughout the entire eruption period, when both ash venting and Strombolian explosions occurred, monochromatic infrasound waves wereobserved nearly every day. Although the peak frequency of the signals (0.4–0.7 Hz) changed over time, the frequency exhibited no reasonable
correlation with the eruption style. The source location of the signals estimated by considering topographic effects and atmosphericconditions was highly stable at the active vent. Based on the findings, we speculate that these signals were related to the resonant
frequencies of an open space in the conduit: the uppermost part inside the vent. Based on finite-difference time-domain modeling using 3-D
topographic data of the crater during the eruption (March 2015), we calculated the propagation of infrasound waves from the conduit.
Assuming that the shape of the conduit was a simple pipe, the peak frequency ofthe observed waveforms was well reproduced by the calculation. The length of
25 Pa
19:50
the pipe markedly defined the peak frequency. By replicating the observed
19:52
waveform, we concluded that the gas exhalation with a gas velocity of 18 m/s
occurred at 120 m depth in the conduit. However, further analysis from a different
19:54
perspective, such as an analysis of the time difference between the arrivals of
infrasound and seismic waves, is required to more accurately determine the
19:56
conduit parameters based on observational data.
19:58

Keywords: Monochromatic infrasound wave, Aso volcano in Japan, The
2014–2015 eruption, Air resonance in the conduit

0

20

40

60

80

100

120

time (s)Graphical abstract*Corresponding author: Akihiko Yokoo, yokoo.akihiko.5a@kyoto-u.ac.jp

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Open Access

Gas flow dynamics in the conduit of Strombolian explosions
inferred from seismo-acoustic observations at Aso volcano,Japan
Kyoka Ishii*, Akihiko Yokoo, Tsuneomi Kagiyama, Takahiro Ohkura, Shin Yoshikawa
and Hiroyuki Inoue
Earth, Planets and Space 2019, 71:13 DOI:10.1186/s40623-019-0992-z
Received: 29 August 2018, Accepted: 24 January 2019, Published: 2 February 2019

Abstract
Strombolian explosions are one of the most studied eruptive styles and are characterized by intermittent explosions. The mechanism of a
Strombolian explosion is modeled as a large gas pocket (slug) migrating through the magma conduit and then bursting at the air–magma
interface. These ascending and bursting processes of the slug induce characteristic seismo-acoustic signals during each explosion: very-longperiod (VLP) seismic signals, eruption earthquake signals, and infrasound signals. However, at Stromboli volcano, it has been reported that theascent velocity estimated from the time differences between observed signals is nearly an order of magnitude higher than that expected fromlaboratory experiments simulating slug ascent. This discrepancy between observation-based and experiment-based velocities has not yet been
fully explained and strongly suggests that the conventional model of Strombolian explosions should be partially revised. In this study, we
attempted to validate the model of Strombolian explosions by estimating the gas phase velocity in the conduit in the case of Aso volcano. We
recorded seismo-acoustic signals accompanying Strombolian events at Aso volcano, Japan, in late April 2015 via our monitoring network, and theascent velocity of the gas phase was determined from the difference in arrival times
between the VLP signals and the infrasound signals. Our estimated velocity exceeded
100 m/s, which is much faster than the experimental value of 7.5 m/s predicted for Aso
volcano. To explain this rapid ascent velocity, we propose a revised model describing
the migration of the gas phase via a more complicated mechanism, such as annularflow. In this model, we assumed that the gas phase ascends in the conduit at highvelocity while making a pathway leading to the magma surface, most likely due to atemporary increase in the gas flux. Our model will help to deepen the understandingof the complicated dynamics in the magma conduit during a Strombolian explosion.

Keywords: Strombolian explosion, Aso volcano, Slug ascent velocity, Seismoacoustic signals*Corresponding author: Kyoka Ishii, ishii.kyoka.46m@st.kyoto-u.ac.jp
11Graphical abstract

Advancement of our knowledge on Aso volcano: Current activity and background

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Open AccessTransport of ballistic projectiles during the 2015 Aso
Strombolian eruptions
Kae Tsunematsu*, Kyoka Ishii and Akihiko Yokoo
Earth, Planets and Space 2019, 71:49 DOI:10.1186/s40623-019-1029-3
Received: 30 August 2018, Accepted: 15 April 2019, Published: 27 April 2019

Abstract
Large pyroclasts—often called ballistic projectiles—cause many casualties and serious damage on people and infrastructures. One
useful measure of avoiding such disasters is to numerically simulate the ballistic trajectories and forecast where large pyroclasts
deposit. Numerical models are based on the transport dynamics of these particles. Therefore, in order to accurately forecast the
spatial distribution of these particles, large pyroclasts from the 2015 Aso Strombolian eruptions were observed with a video camera.
In order to extrapolate the mechanism of particle transport, we have analyzed the frame-by-frame images and obtained particle
trajectories. Using the trajectory data, we investigated the features of Strombolian activity such as ejection velocity, explosion energy,
and particle release depth. As gas flow around airborne particles can be one of the strongest controlling factors of particle transport,
the gas flow velocities were estimated by comparing the simulated and observed trajectories. The range of the ejection velocity ofthe observed eruptions was 5.1–35.5 m/s, while the gas flow
velocity, which is larger than the ejection velocity, reached amaximum of 90 m/s, with mean values of 25–52 m/s for eachbursting event. The particle release depth, where pyroclasts start
to move separately from the chunk of magmatic fragments, wasestimated to be 11–13 m using linear extrapolation of the
trajectories. Although these parabolic trajectories provide us
with an illusion of particles unaffected by the gas flow, theparameter values show that the particles are transported by thegas flow, which is possibly released from inside the conduit.

Keywords: Ballistics, Gas flow, Image analysis, Acoustic energy,
Strombolian eruptions

Graphical abstract*Corresponding author: Kae Tsunematsu, kae_tsunematsu@sci.kj.yamagata-u.ac.jp

EXPRESS LETTER

Open AccessUsing Himawari-8, estimation of SO2 cloud altitude at Aso
volcano eruption, on October 8, 2016
Kensuke Ishii*, Yuta Hayashi and Toshiki Shimbori
Earth, Planets and Space 2018, 70:19 DOI:10.1186/s40623-018-0793-9
Received: 7 September 2017, Accepted: 29 January 2018, Published: 6 February 2018

Abstract
It is vital to detect volcanic plumes as soon as possible for volcanic hazard mitigation such as aviation safety and the life ofresidents. Himawari-8, the Japan Meteorological Agency’s (JMA’s) geostationary meteorological satellite, has high spatial
resolution and sixteen observation bands including the 8.6 μm band to detect sulfur dioxide (SO2). Therefore, Ash RGB
composite images (RED: brightness temperature (BT) difference between 12.4 and 10.4 μm, GREEN: BT difference between
10.4 and 8.6 μm, BLUE: 10.4 μm) discriminate SO2 clouds and volcanic ash clouds from meteorological clouds. Since theHimawari-8 has also high temporal resolution, the real-time monitoring of ash and SO2 clouds is of great use. A
phreatomagmatic eruption of Aso volcano in Kyushu, Japan, occurred at 01:46 JST on October 8, 2016. For this eruption, theAsh RGB could detect SO2 cloud from Aso volcano immediately after the eruption and track it even 12 h after. In this case, theAsh RGB images every 2.5 min could clearly detect the SO2 cloud that conventional images such as infrared and split window
could not detect sufficiently. Furthermore, we could estimate the height of the SO2 cloud by comparing the Ash RGB images
and simulations of the JMA Global Atmospheric Transport Model with a variety of height parameters. As a result of
Himawari-8
comparison, the top and bottom height of the SO2 cloud emitted from the eruption
was estimated as 7 and 13–14 km,
respectively. Assuming the plume height was 13–14 km and eruption duration
was 160–220 s (as estimated by seismic
AshRGB/Band13/Split
observation), the total emission mass of
volcanic ash from the eruption was
estimated as 6.1–11.8 × 108 kg, which isrelatively consistent with 6.0–6.5 × 108 kg
from field survey.
,.,,.

130·

35·

Keywords: Ash RGB, Himawari-8,

Ash RGB
17:45

Atmospheric transport model,
SO2, Aso volcano

Ash RGB
19:45

Graphical abstract
35*Corresponding author: Kensuke Ishii, kishii@mri-jma.go.jp
1235·

Ash RGB
21:45

Earth, Planets and Space

EXPRESS LETTER

Open Access

Aso volcano eruption on October 8, 2016, observed byweather radars
Eiichi Sato*, Keiichi Fukui and Toshiki Shimbori
Earth, Planets and Space 2018, 70:105 DOI:10.1186/s40623-018-0879-4
Received: 5 April 2018, Accepted: 19 June 2018, Published: 28 June 2018

Abstract
An explosive eruption occurred at the Naka-dake first crater of Aso volcano at 1:46 on October 8, 2016 (JST). According to the field survey
conducted by Kumamoto University, Kyoto University, the National Institute of Advanced Industrial Science and Technology, and the Japan
Meteorological Agency (JMA), a large amount of ash fell in the northeast direction of Mt. Aso; in addition, ash falls were confirmed in Kumamoto,
Oita, Ehime Kagawa and Okayama Prefectures. Although the eruption was not observed using a
distant camera, the eruption cloud echoes were captured by five JMA operational weather radars(Fukuoka, Hiroshima, Tanegashima, Muroto-misaki and Matsue). Using these radars, we foundthat the eruption cloud echo moved in the northeast direction at the lower troposphere, and in
the east-northeast direction at the middle troposphere. This result is consistent with the verticalwind shear observed by the JMA wind profiler. After that, the echo of the volcanic ash cloud inthe middle troposphere flowed more than 200 km from Mt. Aso and became ambiguous near
Tosa Bay in Kochi Prefecture. In this study, we introduce a new probabilistic plume height
estimation method. This method probabilistically evaluates errors due to radar beam width andrefractivity of the atmosphere. Using this method, the eruption cloud height is estimated at
12,000 ± 687 m (1σ) above sea level (ASL). This height is lower than 13–14 km, the SO2 cloud
altitude estimated by Himawari-8 Ash RGB and the JMA Global Atmospheric Transport Model andis also lower than the radar echo height indicated by JMA (15 km). However, the plume height
derived using this method is consistent with 39,000 feet (11.9 km) ASL and the volcanic eruption
cloud height analyzed by the Tokyo Volcanic Ash Advisory Center using Himawari-8 infrared
bands. Based on the result of the probabilistic estimation method and the duration of the
volcanic earthquake due to the eruption (160–220 s), the total emission mass was estimated to be
3.2–7.5 × 108 kg, which is almost consistent with the field survey (6.0–6.5 × 108 kg).

Keywords: Weather radar, Remote sensing, Volcanic plume, Eruption cloud, Aso volcano

Graphical abstract*Corresponding author: Eiichi Sato, esato@mri-jma.go.jp

EXPRESS LETTER

Open Access

Temporal variation in the resistivity structure of the first Nakadakecrater, Aso volcano, Japan, during the magmatic eruptions from
November 2014 to May 2015, as inferred by the ACTIVE
electromagnetic monitoring system
Takuto Minami*, Mitsuru Utsugi, Hisashi Utada, Tsuneomi Kagiyama and Hiroyuki Inoue
Earth, Planets and Space 2018, 70:138 DOI:10.1186/s40623-018-0909-2
Received: 5 April 2018, Accepted: 11 August 2018, Published: 23 August 2018

Abstract
During the last magmatic eruption period of Aso volcano (November 2014 to May 2015), a controlled-source electromagnetic volcano
monitoring experiment (ACTIVE) was conducted. Here, we interpret the temporal variations in the electromagnetic responses. The ACTIVE systeminstalled at the first Nakadake crater, the only active crater of Aso, consisted of a transmitter located northwest of the crater and four (before theeruptions) or three (after the eruptions) vertical induction coil receiver stations. The ACTIVE system succeeded in detecting temporal variations in
the resistivity structure during the latest magmatic eruption period. The response amplitude started to increase in November 2014, peaked in
February 2015, and decreased slightly in August 2015. An unstructured tetrahedral finite element three-dimensional inversion that accounted for
topographic effects was used to interpret temporal variations in the ACTIVE response. The 3-D inversion results revealed that temporal variationsin the ACTIVE response are attributed mainly to (1) a broad increase inabresistivity at elevations from 750 to 850 m, not only directly beneath thecrater bottom but also outside the crater, and (2) a thin layer of decrease
in resistivity at the elevation of ~ 1000 m on the western side of thecrater. The increase in resistivity can be ascribed to a decrease in theamount of conductive groundwater in the upper part of an aquifer
located below the elevation of 800 m, while the decrease in resistivity
implies that enhanced fluid temperature and pressure changed thesubsurface hydrothermal system and formed a temporal fluid reservoir
at the shallow level during the magmatic eruption period.

Keywords: Aso volcano, ACTIVE, Resistivity structure, Magma,

Graphical abstract

Eruption, Monitoring, Controlled source*Corresponding author: Takuto Minami, tminami@eri.u-tokyo.ac.jp
13Advancement of our knowledge on Aso volcano: Current activity and background

EXPRESS LETTER

Open Access

Estimation of emission mass from an eruption plumefor the Aso volcano eruption, on October 8, 2016,using a four-dimensional variational method
Kensuke Ishii
Earth, Planets and Space 2018, 70:202 DOI:10.1186/s40623-018-0964-8
Received: 12 September 2018, Accepted: 29 November 2018, Published: 19 December 2018

Abstract
When an explosive eruption, such as a Plinian eruption, occurs, in order to estimate ash fall around the volcano and for hazard mitigation, anumerical model is often used. Simulation by a numerical model needs emission mass from the eruption column including vertical profile andsize distribution of ash particles. Hence, the accuracy of the emission mass from the eruption column is vital to estimate and forecast ash fallaccurately. We developed a data assimilation system based on the four-dimensional variational method (4D-Var) as an estimation method foremission mass from volcanic eruption columns as a function of altitude and ash particle size. This system includes a forward model whichcalculates volcanic ash forecast, and an observation operator, which are used for the calculation of misfit between observation and forecast. It
also includes an adjoint model of the forward model which calculates the correction of emission mass from the misfit, and an algorithm tominimize the cost function as a measurement of optimization. In this system, observation and prior knowledge about emission mass from thevolcanic eruption column, such as the Suzuki function, can be simultaneously treated with weight considering observation error and backgrounderror. Furthermore, this system has scalability for additional observations. That is to say, a variety of observations can be treated simultaneously,
only if their observation operators which are an transformation from model
parameters to observation value are developed. In this study, we applied this system
to the October 8, 2016 Aso volcano eruption in Japan. After this eruption, ash fall
observation (including lapilli) around Aso volcano was preformed, and operational
weather radar captured the eruption cloud echo. Using both of these observations
and the 4D-Var system, we estimated emission mass from the eruption plumecolumn as a function of altitude and particle size, and it led to ash fall simulation
which was consistent with observations. In addition, the eruption mass which is thesum of emission mass from eruption column was estimated to be 1.32 × 108 kg.

Keywords: Data assimilation, Four-dimensional variational method, 4D-Var,
Atmospheric transport model, Aso volcano

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Temporal variations of plume activities before the 8 October 2016
eruption of Aso volcano, Japan, detected by ground-based
and satellite measurements
Masaaki Morita
Earth, Planets and Space 2019, 71:7 DOI:10.1186/s40623-019-0986-x
Received: 31 March 2018, Accepted: 8 January 2019, Published: 16 January 2019

Abstract
At many volcano observatories, measurements of the plume height are frequently applied. On the other hand, the recent
development of the satellite measurements enables the monitoring of the SO2 mass emitted by the passive degassing at
sufficient temporal resolution. Using these two techniques,
this study focuses on the degassing activity before the 8
October 2016 phreatomagmatic eruption of Aso volcano,Japan. Here we show the temporal variations of the plumeheight, the SO2 mass, and ground-based SO2 flux during 6
months before the eruption. Our result shows similar temporalchanges of them, especially for the maxima and the increase,respectively, in about 2 months and 6 days before theeruption. This result indicates that the degassing system had
been stable during the whole study period, but theaccumulation of volcanic gas in the conduit since Augustmight trigger the phreatomagmatic eruption. These
techniques can be sufficient to monitor the degassing activity
and to detect its precursory change.

Keywords: Aso volcano, Volcanic eruption, Monitoring

camera, Volcanic plume, Plume height, SO2, OMPSCorresponding author: Masaaki Morita, aki.morita@aist.go.jp
14

Graphical abstract

Earth, Planets and Space

EXPRESS LETTER

Open AccessHydrothermal system of the active crater of Aso volcano (Japan)
inferred from a three-dimensional resistivity structure
model
Wataru Kanda*, Mitsuru Utsugi, Shinichi Takakura and Hiroyuki Inoue
Earth, Planets and Space 2019, 71:37 DOI:10.1186/s40623-019-1017-7
Received: 31 October 2018, Accepted: 21 March 2019, Published: 10 April 2019

Abstract

log10 Resistivity (Ωm)

CT source region(-fractured, sulfur and sulfate rich)crack-like conduit

geomagnetic source

0 -400

-800

-600

-400

-200

0

200

Easting (m)

b

0
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-1000

groundwater-600

hyper-acidic
fluids

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-1000

groundwater

-800

-600
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high-temperature
volcanic gases

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crater

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1

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Depth (m)log10 Resistivity (Ωm)

3

4001000 800
Graphical
600 400abstract
200 0

crack-like conduit*Corresponding author: Wataru Kanda, kanda@ksvo.titech.ac.jp

-400

500

0

(m
)

-1000

Ea

Keywords: Resistivity structure, Audio-frequency magnetotellurics,

Three-dimensional inversion, Hydrothermal system, AsovolcanoDepth (m)During the past two decades, studies of the Aso volcano in Japan have improved our understanding of the shallow hydrothermal
system that exists beneath the active crater of this volcano. Detailed knowledge of the subsurface structure of this volcanic
edifice is essential for developing a better understanding of the behavior of the volcanic fluids and of the triggering mechanismof volcanic eruptions. Here, we report a three-dimensional (3-D) electrical resistivity model for the active crater of the Nakadakecentral cone of Aso volcano using audio-frequency magnetotelluric (AMT) data obtained during 2004–2005. The AMT data werecollected at 43 sites on a grid (distance between grid points: ~ 300 m) around the crater. However, as yet, only two-dimensional
sectional resistivity models have been generated for this survey area. Using 3-D inversion, we obtain a resistivity model thatshows similar characteristics to those of the 2-D models. A highly
1000
800
600
-500
400
200
0
conductive zone is observed beneath the active crater down to a depth
0-400 -200-1000 -800 -600
500
craterof approximately 300 m. Based on the recent findings regarding theN62Elavas,
3 -1200
S62W-1200
pyroclasts
shallow hydrothermal system of the volcano, we interpret thislavas,
pyroclasts
-1000
-1000
alteredconductive zone to have been formed by highly conductive acidic
2
rocks
fluids filling a fractured region. This view modifies the past
-800
-800
1
interpretation made on the 2-D models and promotes understanding
-600
-600
altered rocks
hydrothermalof fluid behavior beneath the active crater.
reservoir

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Contents
Special issue “Advancement of our knowledge on Aso volcano: current activity and background”. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Takahiro Ohkura, Akihiko Yokoo, Yasuo Miyabuchi, Jerry Fairley, Corrado Cigolini and Valerio AcocellaSalt shell fallout during the ash eruption at the Nakadake crater, Aso volcano, Japan: evidence of an undergroundhydrothermal system surrounding the erupting vent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hiroshi Shinohara, Nobuo Geshi, Akihiko Yokoo, Takahiro Ohkura and Akihiko Terada
Temporal variations in discharge rate and component characteristics of tephra-fall deposits during the 2014–2015
eruption of Nakadake first crater, Aso Volcano, Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yasuo Miyabuchi and Chihoko Hara
The thermal signature of Aso Volcano during unrest episodes detected from space and ground-based measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Corrado Cigolini, Diego Coppola, Akihiko Yokoo and Marco Laiolo
Temporal variation in source location of continuous tremors before ash–gas emissions in January 2014 at Aso volcano,Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Misa Ichimura, Akihiko Yokoo, Tsuneomi Kagiyama, Shin Yoshikawa and Hiroyuki Inoue
Amphibole–melt disequilibrium in silicic melt of the Aso-4 caldera-forming eruption at Aso Volcano, SW Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hidemi Ishibashi, Yukiko Suwa, Masaya Miyoshi, Atsushi Yasuda and Natsumi Hokanishi
Physical characteristics of scoriae and ash from 2014–2015 eruption of Aso Volcano, Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Atsuko Namiki, Yukie Tanaka and Tadashi Yokoyama
Variation of volcanic gas composition during the eruptive period in 2014–2015 at Nakadake crater, Aso volcano, Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hiroshi Shinohara, Akihiko Yokoo and Ryunosuke Kazahaya
Shallow volcanic reservoirs and pathways beneath Aso caldera revealed using ambient seismic noise tomography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yu-Chih Huang, Takahiro Ohkura, Tsuneomi Kagiyama, Shin Yoshikawa and Hiroyuki Inoue
Petrological characteristics and volatile content of magma of the 1979, 1989, and 2014 eruptions of Nakadake, Asovolcano, Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Genji Saito, Osamu Ishizuka, Yoshihiro Ishizuka, Hideo Hoshizumi and Isoji Miyagi
Continuous monitoring of soil CO2 flux at Aso volcano, Japan: the influence of environmental parameters on diffuse
degassing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Masaaki Morita, Toshiya Mori, Akihiko Yokoo, Takahiro Ohkura and Yuichi Morita
Monochromatic infrasound waves observed during the 2014–2015 eruption of Aso volcano, Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Akihiko Yokoo, Kyoka Ishii, Takahiro Ohkura and Keehoon Kim
Gas flow dynamics in the conduit of Strombolian explosions inferred from seismo-acoustic observations at Aso
volcano, Japan. . . . . . . . . . . . . Kyoka Ishii, Akihiko Yokoo, Tsuneomi Kagiyama, Takahiro Ohkura, Shin Yoshikawa and Hiroyuki Inoue
Transport of ballistic projectiles during the 2015 Aso Strombolian eruptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kae Tsunematsu, Kyoka Ishii and Akihiko Yokoo
Using Himawari-8, estimation of SO2 cloud altitude at Aso volcano eruption, on October 8, 2016
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kensuke Ishii, Yuta Hayashi and Toshiki Shimbori
Aso volcano eruption on October 8, 2016, observed by weather radars. . . . . . . . . Eiichi Sato, Keiichi Fukui and Toshiki Shimbori
Temporal variation in the resistivity structure of the first Nakadake crater, Aso volcano, Japan, during the magmatic
eruptions from November 2014 to May 2015, as inferred by the ACTIVE electromagnetic monitoring system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Takuto Minami, Mitsuru Utsugi, Hisashi Utada, Tsuneomi Kagiyama and Hiroyuki Inoue
Estimation of emission mass from an eruption plume for the Aso volcano eruption, on October 8, 2016, using afour-dimensional variational method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kensuke Ishii
Temporal variations of plume activities before the 8 October 2016 eruption of Aso volcano, Japan, detected byground-based and satellite measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Masaaki Morita
Hydrothermal system of the active crater of Aso volcano (Japan) inferred from a three-dimensional resistivity structure
model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wataru Kanda, Mitsuru Utsugi, Shinichi Takakura and Hiroyuki Inoue

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Cover image: Miyabuchi, Y. et al., Earth, Planets and Space 2019, 71:44 DOI:10.1186/s40623-019-1018-6